Recently, sanitary appliances such as sanitary napkins have been introduced with multiple layers of absorbent material. An example of such a napkin is one sold under the trademark NEW FREEDOM Maxi Thin by Kimberly-Clark Corporation, Neenah, Wis. There are advantages to sanitary appliances having multilayer absorbent systems. One of the principle advantages is the ability to design each of the layers of the absorbent to fit particularly desirable product attributes. For example, it is known to be desirable to isolate absorbed fluid from the body surface of the wearer (or user) of a sanitary appliance. It is also desirable to increase the resilience and flexibility of these sanitary appliances to more closely conform to body movements. These goals are accomplished in multilayer pads, at least in part, by having an absorbent layer with relatively large capillaries as the absorbent surface nearest the body of the wearer while utilizing a second absorbent layer of substantially smaller capillaries under the first layer. One of the methods of obtaining an absorbent layer with large capillaries is to form such a layer with relatively large denier thermoplastic fibers alone or in combination with other traditionally absorbent cellulosic fibers as the base for the top layer. U.S. Pat. Nos. 4,082,886; 4,129,132; 3,976,074; 4,054,141; 4,047,531; 3,545,441; and 4,219,024 are representative patents describing the use of thermoplastic fibers in some instances in conjunction with other fibers to form an absorbent web with large capillaries in a multi-component absorbent system.
As the percentage of thermoplastic fibers increase in this upper layer, the amount of fluid retained during use decreases because of the inherent hydrophobicity of these fibers. When the upper layer is made up of a 100% thermoplastic fibers, fluid present in the layer may be easily expelled when subjected to compaction forces. However, for purposes of this invention, the term absorbent layer is meant to include such layers although, they do not function as an absorbent in the traditional sense.
In order to provide a sanitary appliance with a dry surface on the area adjacent the wearer, fluid must be transported rapidly downward away from the wearer and there must be some driving force to provide for this downward, i.e., z direction transfer. While in many instances, gravity provides some of the force necessary for z direction transfer, in the case of viscous fluids, particularly blood and menses, the fluid tends to stay on the surface of the sanitary appliance. In this instance, the difference between the capillary size gradients of the first and second absorbent layer should be as great as practicable. The smaller the capillary size the greater the attraction for fluid and, with regard to the second absorbent layer, the more z direction force which is exerted on the fluid. With regard to the characteristics of the first layer, the larger the capillaries, the less the fluid retention and the more efficient the z direction transfer into the second layer.
U.S. Pat. No. 4,397,644 issued to B. J. Matthews et al describes an absorbent system employing these principles and also localized densified zones in the upper absorbent layer to provide preferential conduit sites for downward transfer due to their reduced capillary size compared to the other portions of this layer.
An effective primary absorbent layer for this system is a surfactant treated meltblown microfiber web described in U.S. Pat. No. 4,372,312 which provides a layer of superior absorbent wicking. This layer is particularly effective when superabsorbent particles or small fibers are entrapped throughout the web. This combination is most efficient when the individual superabsorbent particles or fibers are widely spaced throughout the web. This combination is described e.g. in British application No. 8233488. (Wicking is defined as fluid migration along a surface from the point of initial contact of the fluid.) When an absorbent layer has extremely small capillaries such as the meltblown microfiber and when the capillary structure of the absorbent layer positioned between the wearer and the small capillary absorbent layer has very large capillaries and is extremely thin or when the densified zones are particularly efficient in fluid transfer (as described in the Matthews et al patent hereby incorporated by reference), the rapid influx of the large volume of fluid may overwhelm the system to the extent that the rate of wicking is lower than the rate of fluid discharge on the top surface of the small capillaried absorbent. When this occurs, the small capillary absorbent layer functions as a dam with the fluid backing up into the upper absorbent layer from which will ultimately produce spreading of the stain throughout the upper layer and on the cover material and may actually expel the fluid in some circumstances.
One of the ways to increase the rate of z direction fluid transfer is to eliminate sections of the first absorbent layer entirely. U.S. Pat. Nos. 4,055,180; 3,593,717; 4,342,314; 4,173,046; and British Pat. No. 2,055,586 teach the use of holes either in the cover or in an upper absorbent layer to provide for this increased z direction transfer. This approach does not solve the damming problem associated with the second small capillary-containing layer, however.
The ideal system, therefore, is one which provides for rapid z direction transfer but at a rate which does not overwhelm the horizontal wicking capabilities of the second small capillary-containing layer.